1. Field of the Invention
The present invention relates in general to automatic transmissions for wheeled motor vehicles, and more particularly to a speed change mechanism of the automatic transmissions, which has 6-forward speed and one reverse positions.
2. Field of the Related Art
One of the speed change mechanisms of the above-mentioned type is shown in FIG. 5 of Japanese Laid-open Patent Application (Tokkaihei) 4-219553. The speed change mechanism of this published application comprises generally an input shaft, a planetary gear train including a single pinion type gear unit and two single pinion type gear units, three clutches, two brakes and an output shaft. For achieving 6-forward speed and one reverse portions, the three clutches and two brakes are selectively engaged and disengaged in a given manner.
To provide the speed change mechanism with an overdrive speed position, it is necessary to apply an input force to a pinion carrier and a ring gear of the planetary gear train. However, if this necessity is made in an arrangement wherein the input and output shaft are arranged coaxially, the single pinion type planetary gear, which has only three rotational members, can not provide an input power path for the pinion carrier and the ring gear. For providing such power path, the input and output shafts have to be arranged on different axes which are parallel with each other. However, this arrangement tends to induce a bulky construction of an associated automatic transmission, particularly, the size in a radial direction.
In order to solve such drawback in size, the same published application shows in FIG. 3 another speed change mechanism. In this mechanism, the input and output shafts are arranged coaxially, and in place of the above-mentioned planetary gear train including the two single pinion type gear units, a ravigneawx type complex planetary gear train is used. This gear train has an arrangement wherein two planetary gear units are arranged having their double pinions engaged with respective sun gears.
In this ravigneawx type transmission, one of the planetary gear units has a double pinion type. This means increase in number of portions where gear meshing is made for achieving power transmission. However, increase in number of such portions tends to induce undesirable gear noise and vibration. For preventing such noise and vibration, highly accurate and thus expensive machining and assembling process is needed.
In view of the above-mentioned drawbacks, Japanese Laid-open Patent Application 2001-349390 has proposed in FIGS. 9, 20, 13 and 34, a 6-forward speed and one reverse speed change mechanism which comprises coaxially arranged input and output shafts, a planetary gear train including one speed reduction planetary gear unit and two single pinion planetary gear units, three clutches and two brakes.